Rotate available bridges over time

We need to design a new algorithm for deciding which bridge addresses to give out in response to a given query. The result of this algorithm should be that only a small fraction (e.g. 20%) of the bridges in the https or gmail bucket are available at a given time, and we rotate to a new fraction each week.

Benefits: a) an adversary who does a high-intensity push to enumerate bridges can't get them all in one week now matter how hard the push, and b) if you learned your bridge last week, the enumerating adversary won't find it this week.

This task depends on #1606 (moved) (bridgedb spec), since I don't have a good handle on what we're doing now, so I don't know what needs to change to get there.

And finally, we're going to need to improve Vidalia's "who has used my bridge" interface to explain to the users what's going on, so they don't see zero usage and turn off their bridge.